When considering the potential health risks associated with exposure to toxic substances, it is important to recognize that the response of each individual will not be the same. Efforts to understand the biology of relative susceptibility in humans are thwarted by a number of practical and ethical limitations, therefore it is necessary to develop animal models that can be used for this purpose. One animal model that may be of particular value is the fathead minnow. When exposed to lethal levels of copper, individual minnows that are genetically susceptible to copper die, while genetically resistant individuals survive. If the fish are exposed to sublethal concentrations of copper, all of the minnows acclimate and become resistant to subsequent exposures regardless of their genetic susceptibility. While acclimation occurs in other fish species, the persistence of the response in fathead minnows appears to be extraordinary. Preliminary findings suggest that the response lasts for months, and that it is transferable from parents to offspring. If these preliminary findings are valid, then acclimation may be a valid process that can lead to the formation of resistant subpopulations. The main objective of this project is to statistically validate that the acclimatory response in fathead minnows is long-lasting and can be transferred from parent to offspring. A method has been developed by which we can sublethally evaluate the relative genetic susceptibility of an individual fish while keeping it alive. Using these individuals, the persistence of the acclimatory response will be evaluated in groups of fish tested 1,2,4 and 8 months after the initial acclimation. We hypothesize that the response will be long-lasting (in excess of 2 months) and that the genetic susceptibility of the individuals will not be related to the persistence of the acclimatory response. The transfer of the acclimatory response from parents to offspring will be evaluated from the results of a series of larval survivability tests. The parents of the larvae will be either randomly selected or selected based upon their relative genetic susceptibility. We hypothesize that the results of the larval survivability tests will depend upon the exposure history (acclimation) of the parents, not the relative genetic susceptibility of the parents